1. Field of the Invention
The present invention relates to an electric controller system using a processor such as a micro-computer, and more particularly to a system for controlling industrial process variables such as temperature, pressure and flow rate.
2. Description of the Prior Art
In associating a micro-computer and the like with an analog controller system, it is necessary in the conventional art to provide an analog/digital converter (hereinafter referred to as A/D converter) on the input side of the micro-computer for converting an analog signal to a digital signal and also a digital/analog converter (hereinafter referred to as D/A converter) on the output side for converting a digital signal to an analog signal. The A/D converter is expensive and has a complicated circuit configuration. Hence, if an A/D converter and a micro-computer are employed in an electric controller system, the entire system is complicated and expensive. Further, for manual control of an output signal, the usual practice is to provide a manual control means at the input of each processor to obtain a manual output through a memory in the processor, so as to transfer from automatic control to manual control or vice versa in a balanceless, bumpless manner while avoiding generation of a drift in the output signal during the manual control mode. In such a system, however, a disadvantage is that no manual output is obtainable when the processor fails.